WIP: Doctools migration to Python3

The following is another attempt to move Doctools to Python3. I still have some tests failing because of some missing xml files:

warning: failed to load external entity "/usr/share/sgml/logilab-xml/stylesheet/pre-process/session2prg-cours/root.xsl"
cannot parse /usr/share/sgml/logilab-xml/stylesheet/pre-process/session2prg-cours/root.xsl

Please receive my apologies for the mess. I'm still trying to understand why I was not able to retrieve topics I've previously pushed to the forge.